Studies of ciliary motion serve as model systems for analysis of the molecular mechanisms of microtubule-related cell motility. The complex, 3-dimensional motion of cilia, flagella and spermtails result from 2 distinct but interacting molecular mechanisms. The first generates the force necessary for the sliding filament mechanism and involves active crossbridge formation by the dynein arms. The second mechanism provides the internal shear resistance necessary to convert active sliding into locally regulated, propagated bending and involves, in part, apparently active crossbridge formation between the radial spokes and central sheath. The 2 mechanisms will be studied independently in order to define the molecular basis for each as well as to define their cooperative or feedback interactions. Geometrical analysis of radial spoke behavior in lamellibranch gill cilia (Warner and Satir, 1974) will be pursued in order to further characterize the sliding-related spoke cycles and the manner in which these cycles are related to specific values of active interdoublet displacement and the bend angle. Active sliding is specifically quantal in the bend region, and accordingly, we will study, by both modeling and electron microscopic analysis, the way in which a quantal dynein arm cycle is related to arm position on the A subfiber. Is 1 dynein molecule the equivalent of 1 arm on the A subfiber? We will purify dynein and characterize its subunit composition by SDS gel electrophoresis and electron microscopy and thereby attempt to determine how arm conformation may be related to crossbridging activity.